Exemplary embodiments of the invention are directed to a method for the automatic classification of radar objects by a cluster algorithm for combining a number of object detections, so-called range reports, from successive, coherent integration intervals (also known as bursts).
Signature classification of radar objects is sufficiently known in the prior art and is used in radar technology to reduce false detections and errors in following targets, the so-called tracking. For this purpose, a reception signal is usually processed in an input channel of the radar within a number of successive coherent integration intervals, i.e. bursts, and an object detection is processed using single detections per burst by feeding the same according to the “geometric affiliation” criterion to a cluster algorithm for forming a plot. The single detection is frequently carried out on the radar signal in a two-dimensional matrix in the range Doppler plane, therefore the detections are referred to below as “range reports”. This term is synonymous here for exceeding the threshold value of the signal level in the range Doppler matrix, wherein it is assumed that the radar provides a defined resolution in the “range” dimension.
In the “Doppler” dimension, likewise a defined resolution by the radar is assumed; the individual resolution cells are referred to below as “Doppler filters”. The contents of the Doppler filters are the level of each range cell sorted according to the frequency shift of the radar echo signal. In this case the Doppler filters correspond to different radial speeds of a reflective object. The detection of objects (for example of a helicopter or similar) using the echo signal thereof in the Doppler filters is also a conventional method. In the following, a plot means the output of a detected object, wherein the coordinates of the plot are formed from the centroid of the addresses of the cluster in the coordinate system of the radar measurement (i.e. for example range, Doppler, azimuth). The centroid is referred to below as a plot centroid. The plots are usually input information for the target following (“tracking”), i.e. for the algorithm that tracks the objects against time and, for example, displays this to an observer.
U.S. Pat. No. 6,677,886 is an example of the techniques discussed above and describes a method for the classification of radar objects using Doppler filter amplitudes, wherein range reports are produced and the range reports are combined using a cluster algorithm to form a plot, wherein confidence factors are used to determine whether a cluster represents a real aircraft or a false target.
By incorporating known environmental information as well as specially adapted and high resolution radars, false detections and errors in the tracking of targets can be resolved to a quality determined by the radar design and the ambient conditions. For relatively low resolution radars with high sensitivity, as widely used in civil airspace monitoring, a particularly high number of false detections (=plots of unwanted objects) is to be expected based on the aforementioned usual method for detecting targets, which is not always compatible with requirements on the radar, in that it results in falsely generated or falsely conducted tracks.
Exemplary embodiments of the present invention is therefore directed to recognizing unwanted objects as such and distinguishing the unwanted objects from the target objects (for example aircraft), using a reliable method for the classification of radar objects for low resolution and at the same time for sensitive radar systems.
According to the invention, the aforementioned method for the classification of radar objects is expanded by                adding the Doppler filter amplitudes of the two azimuthally immediately adjacent bursts from the same range cell to each range report;        forming a subset of the received range reports by arranging adjacent range reports from a defined number of bursts sorted according to azimuthal position in a matrix of the dimension azimuth x range with the plot centroid in the center, starting with the range report at the position of the plot centroid;        arranging the Doppler filter amplitudes associated with the range reports from the range plot centroid +/− a defined number of bursts according to the burst sequence; and        forming a combined “plot spectrum” as a vector of range reports from the matrix, which is initiated with the range cell of the plot centroid; then the closest range report in adjacent range cells is inserted for all burst positions for which there is no range report in the range cell. Remaining gaps are filled with the adjacent Doppler filter amplitudes of the range report used and then any gaps still remaining are filled with a suitable noise value.        
The solution according to the invention has proved especially advantageous compared to the prior art because a useful object classification can be carried out even in economically advantageous low resolution radars. In addition, the invention ensures that the resulting “plot spectrum” contains no spectral components of adjacent objects in the context of the selectivity of the plot formation. It is thus possible to significantly reduce false detections both for the objects themselves as well as for the formation thereof into tracks.